Field of the Invention
The present invention relates to a process for melting/sintering powder particles for layer-by-layer production of three-dimensional objects.
Description of the Related Art
The rapid provision of prototypes or small batches is a problem that has frequently been encountered in recent times. Processes that enable this are called rapid prototyping/rapid manufacturing or additive fabrication methods or else just 3D printing. Particularly suitable processes are those in which the desired structures are produced layer by layer, by selective melting and/or consolidation of pulverulent materials. The processes that work according to this principle are referred to collectively by the umbrella term powder bed fusion.
One example of a powder bed fusion process is selective laser sintering (SLS). In this process, powders are briefly selectively irradiated with a laser beam in a chamber, thus melting the powder particles struck by the laser beam. The molten particles coalesce and rapidly resolidify to form a solid mass. By repeated irradiation of a constant succession of freshly applied layers, this process can be used for rapid and simple production of three-dimensional articles.
The process of laser sintering for the purpose of producing shaped articles from pulverulent polymers is described extensively in patents U.S. Pat. No. 6,136,948 and WO 9606881 (both DTM Corporation). WO9208566 describes an annular radiative heating means with which the build region is heated. DE102005024790 A1 describes a radiative heating means with which the build region is rapidly heated with a surface-radiating element, in particular made of graphite.
Further examples of powder bed fusion processes are described in patent specifications U.S. Pat. No. 6,531,086 and EP1740367.
The temperature of the pulverulent polymer in the powder bed is of decisive importance for process safety and the quality of the three-dimensional articles produced by the process. A highest possible temperature of the particles at the surface of the powder bed has the advantage that less energy now needs to be introduced selectively, for example via a laser beam. Additional introduction of energy (post-sintering) after irradiation by the laser for example is then no longer necessary.
In addition, a high temperature of the powder particles at the surface of the powder bed has the advantage that warpage of the just-melted melt layer is minimized. Severe warpage of the melt layer, in particular bending/rolling-up of the edges, is typically described as curling. In order to avoid curling, in particular during processing of polymer material, the temperature of the surface of the powder bed is thus controlled to ensure that warpage/curling is minimized while also ensuring that the powder does not already undergo sintering or melting as a result of the heating. For many polymer powders this process temperature is often only 10-20° C. below the melting point of the polymer powder. A newly applied powder layer should also be heated as rapidly as possible to increase the speed of the build process. In the related art the heating of the surface of the powder bed is therefore effected by means of heat-radiating elements whose radiation has an intensity maximum at a wavelength of about 1400 nm.
In addition to the abovementioned advantages of a high process temperature there are also decisive disadvantages. Aging of the polymer increases dramatically with increasing temperature. It is customary that fabrication using a powder bed fusion process requires many hours. This therefore results in a high level of thermal stress for the polymeric build material. It is a consequence of the related art radiant heating means that deeper layers in the powder bed are also heated by the electromagnetic rays and thus subjected to undesired thermal stress.